The present invention relates to a technique for preventing a deterioration in a discharge lamp and a reduction in a lifetime which are caused by a thermal stress in a discharge lamp lighting circuit for carrying out the lighting control of a plurality of discharge lamps.
For a circuit for lighting a discharge lamp (a metal halide lamp), there has been known a structure in which a DC power circuit, a DC-AC converting circuit and a starting circuit (a so-called starter circuit) are provided. For example, in such a structure that a DC-DC converter is used for the DC power circuit and a full bridge type circuit (which is constituted to set four semiconductor switching elements in two pairs respectively and to carry out ON/OFF control) and a driver circuit thereof are used for the DC-AC converting circuit, a voltage having a positive polarity (or a negative polarity) output from the DC-DC converter is converted into a rectangular wave-shaped voltage in the full bridge type circuit and is then supplied to the discharge lamp.
In order to enhance the lighting property of the discharge lamp, it is preferable that the polarity of a voltage to be supplied to the discharge lamp should be temporarily fixed to provide a DC period (hereinafter referred to as an xe2x80x9cafter-lighting polarity fixing periodxe2x80x9d) immediately after the discharge lamp is lighted up (or it breaks down).
FIG. 20 is a diagram schematically showing the current waveform of a rectangular wave output to be supplied to the discharge lamp.
In the drawing, a time shown in an arrow xe2x80x9cSTxe2x80x9d indicates a time that the discharge lamp is lighted up through a starting pulse, and a period xe2x80x9cTdcfxe2x80x9d having a predetermined width immediately thereafter indicates a first half of the after-lighting polarity fixing period and a period xe2x80x9cTdcrxe2x80x9d indicates a second half (having a reverse polarity to a polarity in Tdcf) of the after-lighting polarity fixing period.
However, the duration of the after-lighting polarity fixing period is to be somewhat limited because a period in which the polarity of a supplied voltage is fixed is increased in such a situation that a lighting state is not immediately brought even if a starting pulse having a high voltage generated by the starting circuit is applied to the discharge lamp, for example. Consequently, the following problems arise.
In the case in which a structure of a bootstrap type is employed for the driving circuit of a semiconductor switching element constituting a DC-AC converting circuit, it is necessary to maintain a quantity of electric charges stored in a capacitor through a power supply. Therefore, the capacity of the capacitor should be set to have a great value.
In a lighting circuit having such a structure that lighting control for a plurality of discharge lamps can be carried out by a common circuit, in the case in which some of the discharge lamps have already been lighted up and the other discharge lamps are to be lighted up, a state in which the polarity of the supplied voltage of the lighted discharge lamp is fixed is continuously maintained for a long period of time when the other discharge lamps are not brought into the lighting state. Therefore, there is a possibility that a bad influence (a reduction in a lifetime or a deterioration) might be caused by the application of a thermal stress to the electrode of the discharge lamp.
Before the discharge lamp is lighted up, similarly, it is preferable that the polarity of the voltage to be supplied to the discharge lamp should be temporarily fixed to provide a DC period (which will be hereinafter referred to as a xe2x80x9cbefore-lighting polarity fixing periodxe2x80x9d). This period is indicated as xe2x80x9cTdcbxe2x80x9d in FIG. 20 (which has the same polarity as that of the period xe2x80x9cTdcfxe2x80x9d). If the duration of the before-lighting polarity fixing period is not limited, the same problems arise.
By placing restrictions in time such that the duration of the before-lighting polarity fixing period and the after-lighting polarity fixing period do not exceed a constant time, the capacity of the capacitor of a bootstrap corresponding to a time limit can be set and the influence of a thermal stress on the discharge lamp can be reduced.
In the method described above, however, an insufficient countermeasure is taken against the thermal stress and the following drawbacks are caused.
Referring to the polarity fixing period permitted for the discharge lamp, the upper bound determined by a product of a current and a time (a so-called current-time product) is set to each polarity in consideration of the lifetime of the discharge lamp. In other words, when a larger current is to flow to the discharge lamp with the polarity fixed, the duration of the polarity fixing period in which the same current is to flow should be reduced.
If it is assumed that the lighting control of two discharge lamps is to be carried out in a circuit, the discharge lamp lighted earlier has a current waveform shown in FIG. 20 when the discharge lamp to be lighted later is lighted up. In the case in which the current-time product has an allowance of 20 to 30 Axc2x7mS (A: ampere, mS; millisecond), it is assumed that the limit range of a duration for the before-lighting polarity fixing period Tdcb is set to 16 mS in consideration of a time taken until the capacitor in the starting circuit is fully charged. If the current value of the discharge lamp lighted earlier is 2.5 A, the current-time product is 16xc2x72.5=40 Axc2x7mS to exceed the allowance.
The cause is that the time limit for the polarity fixing period is uniformly defined. Consequently, it is preferable that the duration of the polarity fixing period (in which the DC lighting is carried out with the polarity fixed) should be determined by a time taken until the product of the time and the value of a current flowing to the discharge lamp has a predefined value. In other words, the duration of the period is reduced when the current value of the discharge lamp is great, and the duration of the period is increased when the current value is small. Therefore, it is possible to dynamically limit the duration of the polarity fixing period corresponding to the state of the discharge lamp. For example, in the case in which two discharge lamps are to be lighted up, a greater one of the values of the currents flowing to the discharge lamps is employed and the duration of the polarity fixing period is limited by a time obtained by dividing a defined current-time product by the current value so that both of the discharge lamps do not exceed the defined current-time product.
However, the first half Tdcf of the after-lighting polarity fixing period arrives subsequently to the before-lighting polarity fixing period Tdcb and the same polarity is set during both periods as shown in FIG. 20, for example (because the lighting state becomes unstable if the polarity is changed when the discharge lamp is lighted up after the arrow ST). Consequently, the same result as that in the extension of the period Tdcb is produced so that a deterioration in the discharge lamp and a reduction in the lifetime are adversely affected. In order to reduce the influence such as a deterioration as much as possible while maintaining the lighting performance of the discharge lamp, it is necessary to set, to the allowance, the current-time-product during the polarity fixing period before and after the lighting.
Therefore, it is an object of the invention to reduce the influence of a thermal stress applied to a plurality of discharge lamps in lighting control for the discharge lamps, thereby preventing a reduction in a lifetime and a deterioration.
In order to attain the object, the invention provides a discharge lamp lighting circuit comprising a DC power circuit for outputting DC voltages having positive and negative polarities and a DC-AC converting circuit for converting an output voltage of the DC power circuit into an AC voltage through a plurality of switching elements and then supplying the AC voltage to a plurality of discharge lamps, wherein in the case in which the discharge lamp is to be lighted up, a polarity fixing period is set to define a polarity of a voltage to be supplied from the DC-AC converting circuit to the discharge lamp before or after the discharge lamp is lighted up or before and after the discharge lamp is lighted up to be positive or negative, and a current flowing to the discharge lamp for the period is controlled so as not to exceed a predetermined limit current value.
According to the invention, therefore, the value of the current flowing to the discharge lamp is restricted for the polarity fixing period in which the voltage to be supplied to the discharge lamp is fixed to one of the polarities before or immediately after the discharge lamp is lighted up. Consequently, a deterioration can be prevented from being caused by a thermal stress applied to the discharge lamp.